1. Field of the Invention
This invention relates to a system for balancing the load in a horizontal axis washing machine.
2. Summary of the Prior Art
Conventional horizontal axis washing machines involve a final spin cycle to extract as much water from washed articles as possible to reduce drying time. However, the requirement of a high spin speed is at odds with quiet operation. At the beginning of a spin cycle the wash load can be quite severely unbalanced, so that when the machine tries to accelerate, noise and vibrations result.
The means that washing machine designers have employed so far to cater for imbalance in the load, is typically to suspend the internal assembly on springs and dampers in order to isolate its vibration. The difficulty is that these suspension assemblies never isolate the vibration completely. As the machine ages, they deteriorate and the problem gets worse. Also, these suspension assemblies require significant internal clearance, and so valuable load capacity is lost when designing a machine to standard outside dimensions. Further, because the internal assembly must still withstand the forces due to the imbalance, considerable extra costs result.
The ideal approach is to eliminate the problem at its source, for which there are various solutions. The first possibility is to ensure that the wash load is evenly distributed prior to spinning. This is an effective solution but it is extremely difficult to achieve in practice. Therefore while steps can be taken to reduce the degree of imbalance that must be catered for, it is not possible to eliminate it sufficiently to ignore it thereafter. Another approach is to determine the size and nature of the load imbalance, and add an imbalance that exactly counteracts the load imbalance.
Methods of compensating for imbalance in horizontal axis washing machines have been disclosed in U.S. Pat. No. 5,280,660 (Pellerin et al.), European Patent 856604 (Fagor, S. Coop). These disclosures relate to the use of three axially orientated chambers running the length of the drum, displaced evenly around the periphery of the drum. When individually filled with water in the appropriate amounts, the chamber can be used to approximately correct imbalances in the axis of rotation.
In our published PCT application WO 00/39382 we described a balancing system for washing machines that compensates for both static and dynamic imbalance. That system is sufficiently accurate that the traditional suspension can be dispensed with.
However, even with that system a load may be sufficiently unbalanced at the early part of the spin cycle that it is not possible to adequately compensate for the imbalance using the active balancing system. Also, different wash items have different water holding properties. As water is extracted in the spin cycle this can lead to a changing imbalance during the spin cycle. Water additions to the balance chambers are not reversible during the spin cycle, so rebalancing leads to an ongoing accumulation until the fill capacity is reached. In combination with a poor initial imbalance, this can lead to occasional inability to reach full spin speed. This in turn requires a fairly strictly controlled initial imbalance, which may in turn require many distribution attempts at the commencement of a spin cycle.